Constant beamwidth antenna

ABSTRACT

An antenna assembly for radio frequency energy is disclosed wherein individual antenna elements making up an array are fed in an improved manner so that the width of a directive beam, or beams, formed by such elements is, or are, maintained substantially constant over a wide band of operating frequencies. Such an operating characteristic is attained by providing frequency sensitive elements, here low pass filters, in circuit with transmission lines feeding selected ones of the antenna elements. The response of such filters is designed progressively to attenuate radio frequency energy to the selected ones of the antenna elements as operating frequency is changed, with the result that the width of the beam, or beams, radiated from the array of antenna elements remains substantially constant over a wide band of operating frequencies.

I United States Patent 11 1 1 1 3,911,442 Hatch Oct. 7, 1975 CONSTANT BEAMWIDTH ANTENNA Primary Examiner-Eli Lieberman [75] Inventor: William B. Hatch, Santa Barbara, Attorney or Firm Philip McFarland; Joseph Calif- D. Pannone; Rlchard M. Sharkansky [73] Assignee: Raytheon Company, Lexington, [57] ABSTRACT Mas$- An antenna assembly for radio frequency energy is [22] Filed: 15, 1974 disclosed wherein individual antenna elements making up an array are fed in an improved manner so that the PP 442,704 width of a directive beam, or beams, formed by such elements is, or are, maintained substantially constant 52 US. Cl 343/754; 343/854 Over a Wide band of Operating frequencies- Such [51] Int. Cl. H01Q 19/06 Operating Characteristic is attained by providing 58 Field of Search 343/854, 753, 754, 755 quency Sensitive elements, here low pass filters in cuit with transmission lines feeding selected ones of [56] References Cited the antenna elements. The response of such filters is UNITED STATES PATENTS designed progressively to attenuate radio frequency energy to the selected ones of the antenna elements as 3,160,887 12/1964 Broussaud et al. 343/854 Operating frequency is changed, with the result that the width of the beam, or beams, radiated from the array of antenna elements remains substantially constant over a wide band of operating frequencies.

3 Claims, 3 Drawing Figures US. Patent 06;. 7,1975 3,911,442

BEAM 0 zOFF 9 F- D 5 BEAM B '3 3 OPERATING BEAM A F/G. FREQUENCY fe BAND f CONSTANT BEAMWIDTl-I ANTENNA BACKGROUND OF THE INVENTION This invention pertains generallyto directive antennas for radio frequency energy and particularly to wide-band directive antennas for radio frequency energy.

It is known in the art that an array of antenna elements may be fed through a parallel platelens, i.e., a

microwave lens, and a plurality of transmission lines in such a manner that one, or more, beams of radio frev adapted to receive radio frequency energy within the same frequency band from one, or more, directions.

In one known antenna assembly of the type just mentioned, a design defining a linear array of antenna elements, transmission lines, microwave lens and a plural-' ity of feedports are formed on a common dielectric substrate using printed circuit techniques. After the so printed dielectric substrate is assembled in operative relationship with one or two ground planes (depending upon whethera microstrip or a stripline assembly is desired), constrained paths in the dielectric substrate are defined for radio frequency energy within a relatively wide frequency band. The dimensions of, and spacing between, thevarious parts of the printed design determine the characteristics of the completed antenna assembly. In particular, with a plurality of feedports along a focal arc, the printed design is so arranged that the electrical'lengths of the paths between each feedport and the antenna elementsare systematically controlled. When allof the feedports are energized, the phase shifts experienced by radio frequency energy passing from each'feedport to the antenna elements are such that a plurality of simultaneously existing beams of radio frequency energy is formed, each pointing :in a different direction. The sameantenna assembly may be operated to form a single one of the beams by simply energizing a single one of the feedports. While such an antenna assembly'is adapted to operation over a wide band of frequencies, experience has proven that the beamwidth of its radiated beam, or beams, varies inverselywith frequency. a

While a variation in beamwidth clue to a change in operating frequency may be-tolerated in many applica tion s, case's exist where such ,a yariationseriouslyafl 'fects proper performance. For example, if (when the larly, if (when the antenna assembly is to produce a single beam) it is desired to reduce clutter when a beam is pointed so as to graze an extendedarea, as the sea or a land mass, itis also,obvious that .any variation .in beamwidth due to a change in operating frequency should be avoided.

SUMMARY OF THE INVENTION Therefore, it is a primary object of this invention to provide an improved antennaassemblyadapted to produce one, or more, beams of radio frequency (or elec tromagnetic) energy, such beam, or each one of such beams, having a beamwidth which .is substantially invariant over a wide band of operating frequencies.

This object and other objects to be discerned are achieved generally, in an antenna assembly wherein an array of antenna elements is disposed to form at least one directive beam, by providing attenuator means in circuit with selected ones of the antenna elements in such an array, individual ones of such attenuator means having frequency response characteristics such that the amplitude taper of the electromagnetic energy across the array is varied as the operating frequency is changed/In a preferred embodiment the attenuator means comprises a number of low pass filters having different cutoff frequencies within a band of operating frequencies, such filters being disposed in circuit with selected antenna elements so that, as operating frequency is increased, the number of energized antenna elements is decreased to maintain the size of the effective aperture of the array at a substantially constant size, measured in wavelengths.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of thisinvention, reference is now made to the following description of the accompanying drawings wherein:

FIG. 1 is a diagram, greatly simplified, of an antenna assembly according to this invention showing the manner in which such an assembly is related to transmitters and receivers in a system, the illustrated antenna assembly being partially broken away to show details of construction of such assembly;

FIG. 2 is a plan view of the dielectric substrate of the antenna assembly of FIG. 1 showing the printed design of the various elements in such antenna assembly; and

FIG. 3 is a graph illustrating how the amount of radio frequency energy passing to each antenna element is controlled as operating frequency is changed.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before referring to the drawings, it should be understood that the beam forrning elements of our preferred antenna assembly are parts of a single stripline or microstrip antenna assembly. That is, a printed circuit defining constrained paths for electromagnetic energy isformed on one side of a dielectric substrate and a metallic ground is formed on the other side of such substrate (if a microstrip antenna assembly is desired), or a dielectric slab is placed over the printed circuit with a second metallic ground plane covering the exposed side of the dielectric slab (if a stripline antenna assembly is desired). For convenience, then, the various parts of the printed circuit will be referred to as though they are, in fact, complete elements. For example, the part of the printed circuit in FIG. 2 defining a plan view of a feedport will be referred to as the feedport itself, it being understood that electromagnetic energy actually passes through the portion of the dielectric substrate (and the dielectric slab, if used) underlying the feedport.

Referring now to FIG. 1, it may be seen that our contemplated antenna assembly 10 may be connected in a conventional manner to a plurality (here three) of transmitters 14a, 14b, 14c and a like plurality of receivers 16a, 16b, through, respectively, transmit/- receive switches 18a, 18b, 18c. The various transmitters and receivers are synchronized by a common system synchronizer of conventional design. Each one of the transmit/receive switches 18a, 18b, 18c is connected to a different one of three feedports 22a, 22b, 220 through lines (not numbered).

The feedports 22a, 22b, 220 are disposed along a focal arc and contiguous with a microwave lens 26 which in turn is connected, through matching sections (not numbered) to a plurality (here eight) of transmis sion lines 28a through 28h to define constrained paths for electromagnetic energy to each one of the antenna elements 24a.....24h. For reasons discussed in detail in US. Pat. No. 3,761,936 entitled Multi-Beam Array Antenna, issued Sept. 25, 1973, directive beams of electromagnetic energy then are formed, as shown, when all of the transmitters 14a, 14b, 146 are energized.

As may more clearly be seen in FIG. 2, low pass filter sections 30a, 30b, 30g and 3011 are disposed integrally with transmission lines 28a, 28b, 30g and 3011. Such sections are well known in the art, being described in the text Microwave Filters, Impedance-matching Networks and Coupling Structures by Matthei, Young and Jones, published 1964 by McGraw Hill Publishing Company, New York, N.Y.. Suffice it to say here that each such low pass filter section here comprises capacitive and inductive portions (as designated by C L C and L in the low pass filter section 30a) whose dimensions may be adjusted to change cutoff frequency. In the illustrated example, as shown in FIG. 3, the cutoff frequencies of low pass filter sections 30a, 3012 are designed to be, approximately, at the midpoint in the band of operating frequencies defined by frequencies 12" and f,,. The cutoff frequencies of the low pass filter sections 30b, 30g are designed to corre spond, approximately, with frequencyf,,. It follows then that as the operating frequency increases from the lower end of the band of operating frequencies, low pass filter sections 30a, 3011 (FIG.2) pass less and less electromagnetic energy until finally (when the operat ing frequency is approximately at the midpoint of the operating band) such filters reach their cutoff frequencies. Thereafter, as the operating frequency increases no electromagnetic energy is passed through the low pass filter sections 30a, 30h and low pass filter sections 30b, 30g (FIG. 2) begin to attenuate. When the operating frequency reaches, approximately,f,,, low pass filter sections 30b, 30g are at cutoff, with the final result that, at f,,, electromagnetic energy may pass only through transmission lines 28c, 28d, 282, 28]". To put it another way, at the lower end of the band of operating frequencies, all antenna elements 24a 24h may be energized, at approximately the midpoint of such band, antenna elements 24b, 24c, 24d, 24e, 24f, 24g may be energized and at approximately the upper end of such band, antenna elements 24c, 24d, 24e, 24fmay be energized. The width of each one of the feedports 22a, 22b, 226 is such that, over the band of operating frequencies, the amplitude taper across the matching sections (not numbered) between the microwave lens 26 and the transmission lines 28a.....28h is substantially constant. Such constancy may, as is known, be achieved by making the width of each one of the feedports 22a, 22b, 22c approximately equal to one-half the shortest wavelength of the radio frequency energy, i,e., one-half the wavelength at f,,.

It will be observed now that as the operating frequency is increased from J}, the amount of radio frequency energy reaching the antenna elements centrally located in the array 10, i.e., antenna elements 24c, 24d, 24e, 24 f; is always the same. As the operating frequency is increased, however, the amount of radio frequency energy reaching the antenna elements located on the edges of the array, i.e., antenna elements 24a, 24b, 24g, 24h, decreases. With low pass filters 30a, 30h having a cutoff frequency approximately equal to the midfrequency in the operating band, and with low pass filters having a cutoff frequency at f,,, the effective size of the aperture defined by energized antenna elements here decreases by a factor of 2 as the operating frequency changes from f to f,,. To put it another way, the size of the aperture, expressed in wavelengths, remains substantially constant when the operating frequency is changed from f to f,,. In this connection, it will be noted that the different feedports 22a, 22b, 22c may, at any instant in time, be energized by radio frequency energy of different frequencies.

Having described one embodiment of my invention, it will now be clear to one of skill in the art that many changes may be made without departing from my inventive concepts. For example, the number of antenna elements and the number of low pass filters may be changed. Thus, if the number of antenna elements is increased to produce a narrow directive beam, the number of pairs of elements associated with low pass filters may also be increased so that beamwidth is held even more constant over a wide band of operating frequencies than in the illustrated example. Further, especially if a single beam (or a plurality of beams having radio frequency energy at a single frequency) is desired, the pairs of low pass filters may be replaced by pairs of radio frequency switches, each pair being actuated at a different frequency within an operating band of frequencies, to reduce the number of energized antenna elements as the operating frequency is changed. It is felt, therefore, that this invention should not be restricted to its disclosed embodiment, but rather should be limited only by the spirit and scope of the appended claims.

What is claimed is:

1. In an antenna assembly for providing a plurality of directive beams of electromagnetic energy, such beams being formed by interference between electromagnetic energy radiated from an array of antenna elements symmetrically disposed about an axis of symmetry, comprising a printed circuit lens having an irregular geometrical shape in selected ones of the different constrained paths between the array of antenna elements and the like plurality of feedports, for varying, in accordance with the frequency of the electromagnetic energy, the amplitude of such electromagnetic energy only in the selected ones of the different constrained paths.

2. The improvement as in claim 1 wherein the selected ones of the different constrained paths are in the outer quarters of the array and the frequency dependent attenuator means in each such path includes a low pass filter.

3. The improvement as in claim 2 wherein the cutoff frequency of the low pass filter in each successive selected one of the different constrained paths, starting from the path to the outermost ones of the antenna elements, is successively higher.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 911 ,442 Dated October 7 1975 Inventor(s)Wi1liam B Hatch It is certified that error appears in the above-identified patent and that said Letters Patentare hereby corrected as shown below:

C01 4 1 48 after the antenna elements in "symmetry," add such array being fed through different constrained paths from a like plurality of feedparts,

Col 4 1 50 after the improvement comprising:

"shape" add Frequency dependent attenuator means, disposed Signed and Sealed this" RUTH C. MASON Altexting Officer C. MARSHALL DANN Commissioner oflatents and Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,911,442 Dated October 7, 1975 O Inventor(s)Wi11iam B. Hatch It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col 4 1 48 after the antenna elements in "symmetry," add such array being fed through different constrained paths from a like plurality of feedparts,

Col 4 1 50 after the improvement comprising: "shape" add Frequency dependent attenuator means, disposed O I d slgnc and Scaled thus- [SEAL] Twenty-first D y f September 1976 Attest: i

RUTH C. N t-SON C. MARSHALL DANN Anestmg off'cer Commissioner uffatenls and Trademarks 

1. In an antenna assembly for providing a plurality of directive beams of electromagnetic energy, such beams being formed by interference between electromagnetic energy radiated from an array of antenna elements symmetrically disposed about an axis of symmetry, comprising a printed circuit lens having an irregular geometrical shape in selected ones of the different constrained paths between the array of antenna elements and the like plurality of feedports, for varying, in accordance with the frequency of the electromagnetic energy, the amplitude of such electromagnetic energy only in the selected ones of the different constrained paths.
 2. The improvement as in claim 1 wherein the selected ones of the different constrained paths are in the outer quarters of the array and the frequency dependent attenuator means in each such path includes a low pass filter.
 3. The improvement as in claim 2 wherein the cutoff frequency of the low pass filter in each successive selected one of the different constrained paths, starting from the path to the outermost ones of the antenna elements, is successively higher. 